For purposes of an exemplary showing, the apparatus and method of the present invention will be described in terms of the rolling of metallic plate. It will be understood that the invention is equally applicable to the rolling of metallic sheets or strips. Furthermore, the workpiece need not necessarily be metallic since the principles of the present invention are applicable to the rolling of any appropriate plastically deformable strips, sheets or plates such as plastics or the like.
Mills for rolling steel and other plates or strips (particularly plates or strips of great width), employing small diameter work rolls to take heavier roll passes, have means provided to minimize deflection of the work rolls when subject to rolling pressure. If this were not the case, the workpiece would lose its flatness.
One way to minimize work roll deflection is to provide large diameter backing rolls. Another is to build the mill as a one-piece, rigid housing in the beams of which are located spaced backing elements to support the work rolls either directly, or through the intermediary or additional rolls. Mills of this type are disclosed, for example, in U.S. Pat. No. 4,295,355.
However, even very large diameter backing rolls will deflect somewhat under rolling load. To compensate for this, they are made barrel-shaped, so that the generant, which happens to be in the roll bite, will stay straight when under rolling pressure and thus preserve an even roll gap all the way across the workpiece (strip). Only when the roll gap is even, will the percentage of reduction in that roll pass be even across the width of the workpiece and, in turn, only in such a case will a flat workpiece stay flat after a roll pass of, for example, 15%.
In the one-piece housing type mills just mentioned, each one of the spaced backing elements is provided with eccentric adjustment means to preserve an even roll gap under rolling pressure. Such adjustment must be precise because even a 0.0001 inch difference in the roll gap will produce a visible wave after a wave-less strip is passed through such a mill.
Applicant has found that a uniform percentage of reduction of the thickness across the workpiece (which is a condition to produce an even elongation during the pass and, consequently, a flat workpiece), can be preserved even better by employing an apparatus and method wherein the rigidity of the backing elements is not relied upon to minimize work roll deflection, but wherein the evenness of the roll gap is assured by providing pressure transmitting elements evenly spaced across the face of the work roll. This approach has an advantage in also being able to control roll pressure across the workpiece so as to compensate for lack of flatness or uneven gauge or temper of the workpiece, and still produce a flat workpiece. If a workpiece is both flat and of even thickness, then all of the spaced pressure transmitting elements must be so adjusted that the roll pressure stays uniform all the way across the workpiece. Consequently, a pass reduction of, for example, 15% will also be uniform and the so-rolled workpiece will stay flat. This is all elementary, but since it represents a complete reversal of accepted practice of design of rolling mills for flat products, it is necessary to look where the adoption of this principle will lead in the design of the mill itself, and what it means to the process of rolling.
Assuming the rolling of steel or other material in plate or strip form, subjected to tension, on a multi-stand tandem mill, following a pass program, usually computer-made (according to the characteristics of the mill, its drive, the material and final gauge), such mill can be built with no screw down at all, and with no indication of the width of the roll gap. The thickness and flatness of the workpiece will be measured after each pass. This is a serious simplification of the equipment and of the control process. Nevertheless, as will be explained below with reference to the drawings, the mill itself (deprived of heavy backing elements), becomes simpler, lighter and less expensive.